1. Field of the Invention
The present invention relates to a porous film, and particularly relates to a hydrolyzable, porous film which comprises a polylactic acid-based resin composition essentially consisting of a lactic acid-based polymer having hydrolyzability in the natural environment and a finely-powdered filler.
The porous film of the invention has high moisture permeability and breathability and is also excellent in flexibility, and is hence suitable for uses such as leak-proof films for a disposable paper diaper and other sanitary materials, packaging materials and filter media. Additionally the porous film is prepared from the polylactic acid-based resin composition and has hydrolyzability. Consequently, the porous film is expected for a countermeasure of waste disposal which has recently been a serious problem.
2. Description of the Related Art
Porous films have already been prepared by blending a specific proportion of an organic or inorganic incompatible matter with polyolefin-based resin, melting, film-forming and stretching the resultant film as disclosed in Japanese Patent Publication Sho 53-12542 and Japanese Laid-Open Patent Sho 56-99242, 57-59727, 60-129240 and 62-138541.
These porous films are mainly used for leakproof films for sanitary materials such as a disposable paper diaper and packaging materials, and are generally applied to so-called throw away uses where the films are abandoned immediately after use.
However, the porous films prepared from polyolefin-based resin cannot be hydrolyzed or have a very slow rate of hydrolysis in the natural environment. As a result, these films remain semipermanently when buried under the ground after use. Disposal of these films in the ocean causes aesthetic damage or destruction of the living environment of marine organisms. Thus, disposal of wastes has become a social problem with expansion in consumption.
On the other hand, polylactic acid and its copolymer have been known as a thermoplastic and hydrolyzable polymer. For example, U.S. Pat. No. 1,995,970 discloses a preparation process of a lactic acid-based polymer by polymerization of lactic acid, lactide or a mixture of these compounds.
These lactic acid-based polymers can be obtained by fermentation of inexpensive materials such as corn starch and corn syrup and can also be prepared from petrochemicals such as ethylene. The lactic acid-based polymer, however, is generally high in hardness and hence has a disadvantage of having low flexibility when used in the form of a film. Consequently, the lactic acid-based polymer has been thought to have many restrictions in use and a porous film of the lactic acid-based polymer has not yet been known.
That is, a porous film consisting of polylactic acid or its copolymer which has hydrolyzability in the natural environment has not yet been known and is a material for providing useful goods in view of the above market demand and protection of the natural environment. Thus, the development of the lactic acid-based porous films has been strongly desired.